1. Field of the Invention
The present invention relates to a transponder of a communication system wherein an interrogator transmits a main carrier, and each transponder which has received the main carrier responds to the interrogator with a reflected wave which is generated by modulating the main carrier in a predetermined manner.
2. Discussion of Related Art
There is known a radio communication system as disclosed in JP-2000-49656A, wherein an interrogator transmits a main carrier to a plurality of transponders, and each transponder which has received the main carrier reflects the main carrier, by modulating the main carrier with appropriate information signal such as an identification signal identifying the transponder, or a subcarrier which has been modulated with such information signal. JP-2705076B2 discloses an example of the transponder used in this type of communication system, which has a high-frequency circuit incorporating an antenna, a quarter-wave line or stub, and a switching element. JP-7-20238A discloses another example of the transponder wherein a matching circuit is provided between the switching element and the antenna, in order to reduce an influence of the impedance of the antenna. In this type of communication system, it is considered to utilize a phase modulation which has a small variation in the modulation factor due to positional variation. JP-3-221890 discloses the use of diodes as the switching elements to generate a modulated signal, and other techniques relating to only the impedance of a load circuit, such as a change in the length of a stub, and selective opening and shorting of the load circuit.